Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), and Computed Axial Tomography (CT) are three medical imaging modalities. PET, SPECT and CT are popular in medicine because of their ability to non-invasively study both physiological processes and structures within the body. To better utilize PET, SPECT and CT, recent efforts have been made to combine either a SPECT scanner with a CT scanner or a PET scanner with a CT scanner into a single system. The combination of PET and CT or of SPECT and CT allows for better registration of the metabolic or functional PET or SPECT images with the anatomic CT image and for improved hospital workflow. The combined scanners share space within the same system housing and share a common patient bed or gurney, but use separate detectors and associated hardware. In the case of the PET/CT scanner, see e.g., U.S. Pat. No. 6,449,331, issued to Nutt, et al., on Sep. 10, 2002, entitled “Combined PET and CT Detector and Method for Using Same,” which discloses a combined PET and CT scanner, and which is incorporated herein by reference in its entirety.
PET and SPECT are nuclear medicine imaging techniques used in the medical field to assist in the diagnosis of diseases. In both cases medical images are regenerated based on radioactive emission data, typically in the form of gamma rays, emitted from the body of a patient after the patient has ingested or been injected with a radioactive substance. PET and SPECT allow the physician to examine large sections of the patient at once and produce pictures of many functions of the human body unobtainable by other imaging techniques. In this regard, PET and SPECT display images of how the body works (physiology or function) instead of simply how it looks (anatomy or structure).
Mechanically, a PET or SPECT scanner consists of a bed (or treatment pallet) and a gantry, which is typically mounted inside an enclosure with a tunnel through the center, through which the bed traverses. The patient, who has been infused with a radiopharmaceutical, lies on the bed, which is then inserted into the tunnel formed by the gantry. In the case of SPECT, the gantry is rotated around the patient as the patient passes through the tunnel. The rotating gantry constrains the detectors and a portion of the processing equipment.
In the case of SPECT, emitted gamma rays are detected from numerous different projection angles by a gamma camera (in most cases, an Anger camera or scintillation camera) about a longitudinal axis of the patient, and converted into electrical signals that are stored as image data. Data from image projections provide a set of images as a result of a process known as image reconstruction. In the case of PET, the scanner detectors are designed to detect simultaneous and oppositely traveling gamma ray pairs from positron annihilation events within the patient. The injected or ingested radiopharmaceutical constrains positron-emitting atoms. The positrons annihilate with electrons in the patient to produce pairs of gamma rays where each member of the pair moves in an opposite direction. The paired gamma rays generate signals when they strike the PET scanner detectors. Signals from the gantry are fed into a computer system where the data is then processed to produce images as a result of a process known as image reconstruction.
PET is considered the more sensitive of the two nuclear medicine imaging techniques, and exhibits the greatest quantification accuracy, of any nuclear medicine imaging instrument available at the present time. Applications requiring this sensitivity and accuracy include those in the fields of oncology, cardiology, and neurology.
Another known tomography system is computed axial tomography (CAT, or now also referred to as CT, XCT, or x-ray CT). In CT, an external x-ray source is caused to be passed around a patient. Detectors on the other side of the patient from the x-ray source then respond to the x-ray transmission through the patient to produce an image of the area of study. Unlike SPECT or PET, which are emission tomography techniques because they rely on detecting radiation emitted from inside the patient, CT is a transmission tomography technique which utilizes a radiation source external to the patient. CT provides images of the internal structures of the body, such as the bones and soft tissues, whereas SPECT and PET provide images of the functional aspects, such as metabolism, of the body, usually corresponding to an internal organ or tissue.
Unlike the pairs of PET scanner detectors required to detect the gamma ray pairs from an annihilation event or the detector heads of the SPECT scanner, the CT scanner requires detectors mounted opposite an x-ray source. In third-generation computed tomography systems, the CT detectors and x-ray source are mounted on diametrically opposite sides of a gantry which is rotated around the patient as the patient traverses the tunnel.
The x-ray source emits a beam of x-rays which pass through the patient and are received by an array of detectors. As the x-rays pass through the patient, they are absorbed or scattered as a function of the densities of objects in their path. The output signal generated by each detector is representative of the x-ray attenuation of all objects between the x-ray source and the detector.
The medical images provided by the SPECT/CT scanner or by the PET/CT scanner are diagnostically complementary, and it is advantageous medically to have images of the same region of a patient from both emission and transmissions scans. To be most useful, the SPECT and CT images or the PET and CT images need to be correctly overlaid or co-registered such that the functional features in the PET images can be correlated with the structural features, such as bones, tumors, and lung tissue, in the CT images. Moreover, an accurate measurement of radiopharmaceutical uptake requires a corresponding accurate measurement of the location and attenuation properties of body tissues that lie along the lines of response used in the PET or SPECT measurement. The potential to combine functional and anatomical images is a powerful one, and there has been significant progress in the development of multi-modality image co-registration and alignment techniques. However, with the exception of the brain, the co-alignment of images from different modalities is not straightforward or very accurate, even when surface markers or reference points are used. To this end, it is desirable to incorporate SPECT and CT scanners or PET and CT scanners into a single gantry, thereby allowing the image data to be acquired sequentially or possibly simultaneously within a short period of time on the same patient table and overcoming alignment problems due to patient movement or internal organ movement such as caused by cancer treatment, respiration, variations in scanner bed profile, positioning of the patient for the scan, and other temporal changes in the patient.
As is well-known, compared to anatomical imaging modalities, SPECT images are photon-limited and generally lack anatomical landmarks, thus making image alignment, and the definition of regions-of-interest, even more of a problem than it is for PET. In addition, nonuniform photon attenuation introduces distortions and artifacts into the reconstructed images. As a result, hybrid CT/SPECT scanners have been developed to address these issues, see e.g., T. F. Lang et al., “A prototype emission-transmission imaging system,” IEEE Nucl. Sci. Symposium Conf. Record 3, 1902-1906 (1991); and T. F. Lang et al., “Description of a prototype emission transmission computed tomography imaging system,” J. Nucl. Med. 33, 1881-1887 (1992). In such a hybrid system, it is suggested to use the X-ray CT image to provide the attenuation factors to correct the SPECT data, see e.g., J. S. Fleming, “A technique for using CT images in attenuation correction and quantification in SPECT,” Nucl. Med. Commun. 10, 83-97 (1989). The use of CT images for attenuation correction had been originally proposed by S. C. Moore, “Attenuation compensation” in Ell, P. J. et al., Computed Emission Tomography, London, Oxford University Press, 339-360 (1982). The 100 kVp X-ray source is capable of producing a dual-energy X-ray beam, such that an energy-corrected attenuation map can be obtained for use with the radionuclide data, as disclosed by B. H. Hasegäwa et al., “Object specific attenuation correction of SPECT with correlated dual-energy X-ray CT,” IEEE Trans. Nucl. Sci. NS-40 (4), 1242-1252 (1993). Operating the device with two energy windows also allows simultaneous emission-transmission acquisitions to be performed, although the authors report a certain level of contamination of the emission scan by the transmission X-ray beam. This disclosure demonstrates the potential of a device capable of performing both anatomical and functional measurements. It has also given rise to a detailed simulation study to investigate the different techniques for scaling the attenuation coefficients from CT energies (50-80 keV) to SPECT (140 keV). See K. J. LaCroix et al., “Investigation of the use of X-ray CT images for attenuation compensation in SPECT,” IEEE 1993 Medical Imaging Conference Record (1994). Similarly, CT images can be used for attenuation correction in PET images. (See, e.g., U.S. Patent Application 2004/0030246 by Townsend et al., which is incorporated in its entirety herein).
While the attenuation correction for PET is of a greater magnitude than for SPECT, it is theoretically more straightforward. However, since it is generally based on patient measurements (a transmission scan), it introduces additional noise into the reconstructed emission scan. In practice, in order to limit the duration of the PET scan procedure, abdominal transmission scans of 10-15 minutes are typical, during which 100 million counts are acquired (3 million per slice, or 100 counts per coincidence line of response, i.e. a 10% statistical accuracy), which introduces significant noise into the corrected emission scan. In practice, only lines-of-response (LOR's) through the patient constrain useful transmission information, and since some of the coincidence events will lie in LOR's which do not pass through the patient, the total useful counts in a transmission scan is often less than 100 million. In addition, patient movement between the transmission and emission scan (which may be acquired 40 minutes or so later) can introduce serious artifacts and distortions into the reconstructed image, as disclosed by S. C. Huang et al., “Quantitation in positron emission tomography: 2. Effects of inaccurate attenuation correction,” J Comput Assist Tomogr 3, 804-814 (1979).
Inaccurate attenuation correction can increase when a patient extends beyond the field of view during CT measurements. For example, obese patients can receive cardiac PET/CT scanning procedures. As shown in FIG. 1, a CT section of an obese female is shown with the anatomy being well visualized within the FOV of the CT with gray values being fairly quantitative. However, outside the FOV of the CT, the edges are mis-positioned and CT values can be in error, which can lead to errors in the PET attenuation correction. The arrows in FIG. 1 point to regions with a pronounced truncation artifact. Typically, these problems are not present when the patient is within the FOV of the CT as shown in FIG. 2. To compensate for the problems associated when a patient is not within the FOV of the imaging system, software can be used to reduce the errors, e.g., extended field of view reconstruction method (EFOV). Although, the software assists in reducing the errors, the corrections only approximates the actual image outside the FOV and degrades the more a patient extends further outside the FOV.